The spheroidizing treatment process of nodular cast iron mainly includes the types of spheroidizing agent and the adding method of spheroidizing agent. The types of spheroidizing agents mainly include magnesium series, rare earth series and calcium series. These three kinds of spheroidizing agents have their own characteristics and need to be selected flexibly according to the actual production situation.
Magnesium spheroidizing agent for nodular cast iron is one of the most widely used spheroidizing agents. Magnesium is the element with the strongest spheroidizing ability, which can make the treated graphite ball very round. Due to the strong deoxidation and desulfurization ability of magnesium, the sulfur content of raw molten iron can be appropriately relaxed during use, and a good spheroidizing effect of nodular cast iron can also be achieved in Hypoeutectic or hypereutectic composition. Magnesium spheroidizing agent also has obvious disadvantages. The anti-interference ability of magnesium spheroidization is poor. Due to the low density and low boiling point of pure magnesium, it is very easy to burn when added, resulting in a large amount of white smoke and flash, resulting in environmental pollution.
The spheroidizing elements of rare earth spheroidizing agent are mainly light rare earth cerium and heavy rare earth yttrium. The characteristic of rare earth spheroidizing agent is that the anti-interference ability of spheroidizing agent is stronger than that of magnesium. The boiling point of rare earth elements is high, and there will be no boiling, flash and smoke after adding, which can make the spheroidization reaction of nodular cast iron very stable and cause little pollution to the environment. The disadvantage of rare earth spheroidizing agent is that the spheroidizing effect of nodular cast iron is poor, and the obtained graphite ball is not round enough. Rare earth elements have white tendency and are prone to segregation. If the amount is too high, it will deteriorate the mechanical properties of nodular cast iron, especially the plasticity and toughness.
Calcium spheroidizing agent is a kind of spheroidizing agent which is less used. The characteristic of calcium spheroidizing agent is that the white tendency of calcium is smaller than that of magnesium, and it is less sensitive to the content of silicon and manganese. It is easy to obtain ferrite in as cast state, and the spheroidizing reaction of nodular cast iron is stable. The disadvantage is that the spheroidizing ability of calcium is weak, the spheroidizing effect of nodular cast iron is not good, and flake graphite often appears in the gold phase. The solubility of calcium in molten iron is very small, which is significantly affected by temperature, and the addition amount is not easy to grasp. Calcium spheroidizing agent is easy to oxidize and fail in air and cannot be stored for a long time.
The selection of spheroidizing agent for nodular cast iron is closely related to the composition of raw materials used in production. In foreign countries, because the raw materials are relatively pure, the chemical composition of molten iron is relatively single, and there are no a large number of interfering spheroidizing elements, magnesium or low rare earth magnesium spheroidizing agents can be used to obtain better spheroidizing effect. Domestic raw materials often contain more interference elements. Generally, the selection of pure magnesium spheroidizing agent can not obtain the ideal spheroidizing effect of nodular cast iron. Therefore, it is necessary to add an appropriate amount of rare earth elements to nodular cast iron spheroidizing agent to improve the anti-interference ability of nodular cast iron spheroidizing agent, so as to ensure the spheroidizing effect.